extra 7th axis motor setup on kuka krc4

sorry...no way with that little info...

sizing motor/gearbox takes some effort.

knowing motor type determines cable size but nothing else - such as length or version (standard or high flex - this is application dependent and costs vary).

you also need to check if your controller has suitable drive and motor interface or reconfiguration is needed.

RDC supports up to 8 resolvers and since you only have robot, up to two more resolvers can be connected - question is where that external axis motor is going to be mounted because on Quantec robots RDC is moving with A1. if this motor is to be on the robot arm (for example part of EOAT) then simple resolver cable with strain relief and dowel pin is all you need. if external axis is NOT mounted on the robot, you need two cables, one is from RDC to base of the robot, other is from base of the robot to motor and they will have different connections.

i suggest to get in touch with Kuka...

you should have a CD inside the controller. that is the product documentations and spare parts. that includes robots and cables that you need.